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COMPUTER CONTROLLED FABRY PEROT INTERFEROMETERS
Augustinus Asenbaum

Institute for Physics and Biophysics

University of Salzburg, Austria



ABSTRACT
A review is given about the development of stabilizing Fabry-Perot Interferometers with the aid of intelligent tools, finally with computer control. The interferometer can be used as a very high resolution spectrometer. It can also serve as a laser resonant cavity. Basically it consists of two highly reflecting plane and parallel mirrors with an air gap of a few cm between them. For nearly all applications of the instrument a perfect alignment of the instrument is necessary. That means to control the separation between the mirrors within a few 10-10 m during a scan of a few 10-6 m and also to maintain the parallel position of the plates within a few 10-10 m. Furthermore a long term stability of the parallel alignment of the instrument is necessary to record weak scattered light signals. In 1961 the first automatic control of the mirror parallelism was achieved with intelligent shutters, switches, sample and hold circuits etc. In 1975 the first computer controlled instruments were developed. The introduction of multi-pass Fabry-Perot interferometers by Sandercock to increase the contrast of the instrument allowed the recording of Brillouin spectra of solid samples. New developments include a very sophisticated apparatus with computer control and easy to handle software, so not only highly experienced scientists are able to run a tandem Fabry Perot interferometer to perform Brillouin scattering experiments.

Fabry-Perot interferometers are often used to measure spectral line shifts and widths with high precision. For weak light scattering signals it takes a long time to get a quite good signal-to-noise ratio. A free running Fabry Perot can hold a nearly perfect alignment only for a couple of seconds.

Therefore it is necessary to adjust the Fabry Perot interferometer automatically during each scan for optimal parallelism. The deviation of the mirrors from ideal parallelism should not exceed 5x10-10 m (1).

Also during a scan the parallel alignment of the mirrors should not change, because such a misalignment would also change the resolution and the contrast of the instrument... For multi-pass instruments during a scan different peak intensities of the different orders could be seen. Considerable work was done to overcome these difficulties (7-35). Ramsay (7) designed a system with an automatic control of the mirror parallelism with the aid of two white light sources. Further work was done by Smeethe and James (9), Hernandez and Mills (10), Winter (11) and Jacka et al. (12) with the aid of Ramsay's method.

Servo controlled Fabry Perot interferometers were designed by Hicks et al. (13-15) and Rees et al. (16). The parallel position of the mirrors and the plate separation were checked by four small capacitances.

Sandercock (17) was using the elastic scattered laser line to monitor finesse and the plate separation of a Fabry-Perot interferometer during scanning. One mirror could be moved with the aid of piezoelectric crystals (18) around two mutually orthogonal axes. Taking the elastically scattered light as a reference line a multichannel analyzer (MCA) in multiscaling mode was used to sum up the single spectra and cancel out slow fluctuations of in the laser wave length and longitudinal drifts of the Fabry Perot cavity.

The next step was to introduce multi-pass Fabry Perot interferometers (17-20) using this method (21-25, 27). The very high contrast of this instrument allows the recording of Brillouin spectra of solids, in spite of the high intensity of unwanted stray light due to surface imperfections of the solid samples. The alignment of multi-pass instruments with switched off stabilisation is decreasing in a few minutes (26).

Computer stabilized Fabry-Perot setups were reported by Hewko and Torrie (28) and Wood (29).

In 1979 a Fabry-Perot system controlled by a PDP-11 computer was developed by Asenbaum (30), where for the first time the half width of the instrumental function was used to check the parallel adjustment of the mirrors. A similar arrangement was reported by Yamada et al. (31) in 1980.

In 1980 the first tandem systems were designed to increase the effective finesse of a Fabry-Perot by about a factor of 10 (32-34).

A tandem Fabry-Perot using two separate triple pass interferometers was reported by Dil et al. (33). A five-pass and a four-pass tandem Fabry-Perot was developed by Lindsay et al. (34). Mock et al. (35) got a contrast of 1012 using a tandem interferometer in a single triple-pass mode. A comprehensive overview about designs of Fabry-Perot interferometers is given by Vaughan (36) and Hernandez (37).

Aschauer et al. (38) designed a computer controlled Fabry-Perot system for both single and multipass operation, where the test and correction steps for the mirror adjustment were performed around two independent axes perpendicular to each other.

A computer controlled multipass tandem Fabry-Perot was demonstrated by Hillebrands (39), which has advantages over analog control. The range of stability is increased due to active control of the laser intensity and the mirror dither amplitude.

The sophisticated software allows to control the position and rotation of the sample, the angle of light incidence, the sample temperature or the direction and strength of an external field.

Finally fitting routines make it possible to correct residual nonlinearity of the mirror stage drive.

Further work is necessary to close the gap between high resolution optical and electronic spectrometers under computer control.

The author would like to thank the Jubiläumsfonds der Österreichischen Nationalbank project number 7653.
REFERENCES
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39. B. Hillebrands, “Progress in multipass tandem Fabry-Perot interferometry: I. A fully automated, easy to use, self aligning spectrometer with increased stability and flexibility”, Rev. Sci. Instrum. 70, 1589 (1999)







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